Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
6.0 Mechanical Redesign – Depth
In an attempt to reduce energy consumption costs for the Struamann USA facility,
several mechanical system alternatives will be compared. On the air-side of the
mechanical system a dedicated outdoor air system (DOAS) with a parallel radiant
cooling panel system will be compared to a variable air volume (VAV) system. Two
different chiller types will be explored on the waterside, electric centrifugal and direct fire
absorption. Two different piping arrangements will also be explored for taking
advantage of free cooling, parallel and series.
6.1 Air Systems
The airside analysis of the Straumann facility will compare a common VAV system with
a combination DOAS and radiant cooling panel system. VAV systems are probably the
most popular type air system installed in the United States. While they have become
very popular in buildings, there are other types of systems that can also be explored.
When comparing a VAV and DOAS systems, the are advantages to implementing both
systems.
A VAV system, as seen in Figure 6.1-1, is capable of providing both ventilation air and
thermal cooling all from the same air system. A DOAS system, shown in Figure 6.1-2,
typically provides ventilation and latent cooling from a smaller air system and must be
coupled with a separate parallel system, in this case radiant panels, in order sensibly
cool a building. DOAS air handling units are smaller than those required by a VAV
system since DOAS units are usually only supplying air to meet minimum ventilation
requirements. Often the DOAS unit will supply slightly more air than required by
minimum ventilation standards in order to provide latent cooling for spaces. This
prevents condensation from becoming a problem with any parallel systems like radiant
panels.
- 13 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Figure 6.1-1: VAV System
Schematic
Figure 6.1-2: DOAS System
Schematic
Several advantages result in a DOAS system due to the reduced air handling unit size.
Since less air is required, smaller fans are used and annual fan energy, which can be a
major portion of the annual mechanical operating costs, is reduced. For the Straumann
USA facility, fan energy accounts for nearly 30% of the annual electric costs for the
mechanical systems. A reduction in fan energy has the potential to result in significant
annual operating costs. Another benefit is that a DOAS unit typically supplies less
ventilation air than required in a VAV system. This results in a lower energy
requirements to condition the ventilation air.
When using a parallel radiant cooling panel system with DOAS, it will result in higher
pumping costs than associated with VAV systems. In order to sensibly cool a space,
chilled water is pumped to panels above the ceiling where it radiantly cools the occupied
spaces. Even though increased pumping energy costs are seen, the reduction in fan
energy costs are typically higher, which still results in the DOAS system reducing
annual energy costs.
While it may seem that DOAS systems will save yearly energy, it is important to
compare not just yearly energy costs, but the first cost of the systems as well. In order
to determine the best system for Straumann USA, an energy analysis and first cost
comparison for major equipment will both be taken into consideration before making a
recommendation.
Carrier’s Hourly Analysis Program will be used to calculate the annual energy costs
associated with both a VAV and a DOAS system for Straumann USA. Table 6.1-1 lists
the design conditions that will be applied both systems.
- 14 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Load Analysis Assumptions
OA Ventilation Rates
ASHRAE Standard 62.1-2004
Lighting Loads
Office
1.3 W/ft2
Manufacturing
2.2 W/ft2
Equipment Loads
Office
3.0 W/ft2
Manufacturing
38W/ft2
Design Conditions
ASHRAE Fundamentals 2005 (0.4%)
Summer
Dry Bulb
90.8
Mean Coincident Wet Bulb
73.1
Winter
Dry Bulb
7.7
Table 6.1-1: Design Assumptions
In order to perform the analysis for the DOAS system new zones must be selected for
the DOAS rooftop units. The new DOAS zones are displayed in Figure 6.1-3 and Table
6.1-2 gives a brief description of each. Similar figures and descriptions for the VAV
system is found in section 4.7 Existing Mechanical Conditions. The DOAS rooftop units
are designed around the Carrier Centurion packaged DX rooftops units, but any
equivalent DX rooftop unit could be used. The radiant panels are designed around the
Barcol-Air REDEC-CB radiant panel which has a cooling capacity of up to 54 Btu/ft2. An
initial estimate of loads and sensible cooling capabilities of the radiant panels
determined a DOAS and radiant panel system would not work in the manufacturing
area. Therefore, a VAV system will continue to be utilized in this area being served by
RTU-5,6,7,8. The DOAS system considered will be a combination VAV system for the
manufacturing area and a DOAS system for the remainder of the facility. For the
design, the dew point is at 55°F so the mean radiant temperature based on the sterling
design guide will be 56.5°F. The chilled water supply and return temperatures to the
radiant panels will be designed using 54°F/59°F.
RTU-1
RTU-2
RTU-3
RTU-4
RTU-5
RTU-6
RTU-7
RTU-8
DOAS Rooftop Unit Summary
Square Feet Areas Served
Max CFM
Served
4,273
41,993
First floor manufacturing support areas
First floor dental operatory and
3,328
38,549
mezzanine office areas
1,052
4,885
First floor auditorium
3,089
23,361
First floor office and lobby areas
33,000
5,850
Manufacturing area
33,000
5,850
Manufacturing area
33,000
5,850
Manufacturing area
33,000
5,850
Manufacturing area
Table 6.1-2: Spaces Served by Each DOAS Rooftop Air Handling Unit
- 15 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Figure 6.1-3: DOAS Rooftop Air-Handling Unit Zones
The amount of ventilation air introduced into the building decreased by over 50% from
the VAV to DOAS systems. ASHRAE Standard 62.1-2004 ventilation rates are still met
by the DOAS system. The reason for the increased ventilation requirements for the
VAV system is that the critical space ventilation requirement must be met in each zone.
This typically results in other spaces being over ventilated. Supplying additional
ventilation air is not a problem but does require more energy to condition. By using a
DOAS system, each space is supplied with the exact amount of required ventilation,
and is not over ventilated. Table 6.1-3 summarizes the amounts of both supply and
ventilation air for the two systems.
Ventilation Air
Supply Air
VAV (CFM)
DOAS (CFM)
35,144
510,400
15,104
143,742
% Reduction by
DOAS
57.0%
71.8%
Table 6.1-3: Ventilation and Supply Air Comparison
Annual energy and cost estimates are listed in Table 6.1-4 and Table 6.1-5 respectively.
As expected, the DOAS system significantly reduces the amount of fan energy for
- 16 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Straumann USA. The cooling required for the facility also decreased probably due to
the reduction in ventilation air conditioning. The heating energy is also decreased. This
is an unexpected benefit but could be a result of supplying a lower minimum air flow to
each space which would result in less required reheat at low occupancy conditions.
The only increased cost is pump energy and that is to be expected when supplying
chilled water to radiant panels rather than just rooftop air-handling units. Overall, this
analysis shows that a DOAS system will result in annual energy and cost savings for
Straumann USA.
Energy (MMBTU)
Electric Centrifugal Chiller
Component
Straumann
Straumann VAV
DOAS/VAV
Air System Fans
1,564
1,093
Cooling
1,229
1,202
Heating
1,250
616
Pumps
356
455
Cooling Tower Fans
156
155
HVAC Sub-Total
4,554
3,521
Lights
1,509
1,509
Electric Equipment
9,326
9,326
Non-HVAC Sub-Total
10,835
10,835
Grand Total
15,389
14,356
DOAS Savings
471
26
634
(99)
0
1,032
0
0
0
1,032
Table 6.1-4: Annual Energy Comparison
Component
Air System Fans
Cooling
Heating
Pumps
Cooling Tower Fans
HVAC Sub-Total
Lights
Electric Equipment
Non-HVAC Sub-Total
Grand Total
Straumann VAV
$72,647
$64,415
$42,958
$17,916
$8,961
$206,897
$68,570
$423,845
$492,415
$699,312
Cost
Straumann
DOAS/VAV
$50,727
$62,839
$20,298
$24,035
$8,752
$166,651
$68,570
$423,845
$492,415
$659,066
DOAS Savings
$21,920
$1,576
$22,660
($6,120)
$209
$40,245
$0
$0
$0
$40,245
Table 6.1-5: Annual Cost Comparison
6.2 Central Plant Systems
Although the central plants were not replaced at the 100 Minuteman building at during
the renovation work for Straumann USA, a few options will be explored in this report.
- 17 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
The first potential area for energy savings will be explored in comparing electric
centrifugal chillers with direct fire absorption chillers. The second area that will be
explored for energy savings will be comparing the possibility of changing piping
arrangement of the waterside free cooling from a parallel to a series design.
6.2.1 Chiller Options
Currently water cooled centrifugal electric chillers provide chilled water for the
Straumann USA facility. While the renovation of the central chilled water plant is not a
part of the original project, it is possible that a change in chillers could provide a
reduction in energy savings. This analysis will compare the effects of replacing the
current chillers in kind with the possibility of replacing the chillers with direct-fired
absorption chillers.
While absorption chillers typically have a lower COP than electric chillers, an absorption
chiller can save energy under the right circumstances. A sample of an electric
centrifugal and absorption chiller is displayed in Figure 6.2-1 and Figure 6.2-2
respectively. Steam driven absorption chillers can take advantage of large process
loads that may need to reject heat to power an absorption chiller. Utilizing district steam
to power an absorption chiller is yet another way to reduce electric costs. Unfortunately,
there is no district steam or large process loads available on site in order to use a steam
driven chiller. This limits the analysis to a direct-fired absorption chiller, which will be
powered by natural gas already located on site.
Figure 6.2.-1: Trane Electric
Centrifugal Chiller
Figure 6.2-2: Carrier
Direct-fired Absorption Chiller
Both electric vapor compression, and absorption chillers provide cooling through
condensing and evaporating a refrigerant. Electric chillers mechanically change the
pressure of the refrigerant with a compressor while an absorption chiller utilizes a
sorption and desportion process instead of a compressor to achieve the same effect.
An electric vapor compression cycle is displayed in Figure 6.2-3. In this type of chiller
the refrigerant is heated in the evaporator by the warm chilled water return. An electric
compressor then increases the pressure of the refrigerant. Next the refrigerant flows
- 18 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
through the condenser where it is cooled by supply water from the cooling tower or
other condenser water source. The cycle is completed when the cooled refrigerant
passes through an expansion valve reducing the pressure and reentering the
evaporator.
Figure 6.2-3: Vapor Compression Cycle
A double effect absorption chiller is a slightly more complicated process and is
displayed in Figure 6.2-4. The process starts once again in the evaporator where it is
heated until it becomes a vapor by the warm returning chilled water. The refrigerant
than travels into the absorber where it condenses and is mixed with an absorbent. The
heat generated in the absorber is removed by the condenser water. The mixture of
absorbent and refrigerant is pumped to the low generator. Here some heat is added
from the high temperature refrigerant vapors leaving the high generator. This boils
some of the refrigerant out of the mixture in the low generator. Some of the mixture of
refrigerant an absorbent left in the low generator is mixed with the absorbent returning
from the high generator and is sprayed back into the absorber. The rest of the mixture
in the low generator is pumped to the high generator. In the high generator heat from
burning natural gas boils off the remaining refrigerant which passes into the condenser.
The absorbent does not evaporate and travels back to the absorber being cooled along
the way by preheating absorbent and refrigerant mixture that is entering both the high
and low generators. The refrigerant that entered the condenser in the form of vapor is
cooled back to a liquid by condenser water and then passes through an expansion
valve before re-entering the evaporator.
- 19 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Figure 6.2-4: Double Effect Absorption Cycle
Once again, Carrier’s Hourly Analysis Program will be used to perform annual energy
analysis. The new absorption chiller will be designed using the same design conditions
as the original electric chillers. Condenser water temperatures are 85°F/95°F and
chilled water is designed to supply 45°F chilled water. The designs will be based
around Carrier’s double effect Centurion direct-fire absorption chiller and Trane’s
EarthWise CenTraVac electric centrifugal chiller. However, any chiller of comparable
performance could be used. Both of the previously discussed airside systems will be
considered with each type of chiller. Annual energy and cost results are summarized in
Tables 6.2-1 and 6.2-2 respectively.
Energy (MMBTU)
Direct-fired Absorbtion Chiller
Electric Centrifugal Chiller
Component
Straumann
Straumann
Straumann VAV
Straumann VAV
DOAS/VAV
DOAS/VAV
Air System Fans
1,564
1,093
1,564
1,093
Cooling
1,229
1,202
5,838
5,072
Heating
1,250
616
1,250
616
Pumps
356
455
439
542
Cooling Tower Fans
156
155
246
146
HVAC Sub-Total
4,554
3,521
9,337
7,468
Lights
1,509
1,509
1,509
1,509
Electric Equipment
9,326
9,326
9,326
9,326
Non-HVAC Sub-Total
10,835
10,835
10,835
10,835
Grand Total
15,389
14,356
20,172
18,303
Table 6.2-1: VAV and Absorption Chiller Annual Energy Comparison
- 20 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Cost
Electric Centrifugal Chiller
Straumann
Straumann VAV
DOAS/VAV
Component
Air System Fans
$72,647
$50,727
Cooling
$64,415
$62,839
Heating
$42,958
$20,298
Pumps
$17,916
$24,035
Cooling Tower Fans
$8,961
$8,752
HVAC Sub-Total
$206,897
$166,651
Lights
$68,570
$68,570
Electric Equipment
$423,845
$423,845
Non-HVAC Sub-Total
$492,415
$492,415
Grand Total
$699,312
$659,066
Direct-fired Absorbtion Chiller
Straumann
Straumann VAV
DOAS/VAV
$72,647
$50,727
$107,264
$92,452
$42,958
$20,298
$21,720
$28,737
$13,779
$9,055
$258,368
$201,270
$68,570
$68,570
$423,845
$423,845
$492,415
$492,415
$750,783
$693,685
Table 6.2-2: VAV and Absorption Chiller Annual Cost Comparison
The energy analysis of centrifugal and absorption chillers provided some interesting
results. When comparing similar airside systems an absorption chiller consumes more
energy and is more expensive annually. However, on an annual cost basis, using a
DOAS airside system and an absorption chiller, it is actually cheaper than the electric
chiller with a VAV system. When comparing the amount of energy consumed for these
two system the opposite is true, the electric chiller and VAV system actually consumes
less energy.
It is possible to use the use the absorption chillers for simultaneous heating and cooling
which could also result in energy savings. Rather than using a separate boiler system
to provide heating, the chiller might be able to provide both hot water for perimeter fintube radiators as well as well as chilled water for radiant panels. The chiller heater
option with absorption chillers depends largely on the heating and cooling load profiles.
The amount of heating a chiller heater can produce depends on the amount of cooling
the chiller is performing. Figure 6.2-5 displays the give and take effect of the heating
and cooling capabilities of a chiller heater.
- 21 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Figure 6.2-5: Heating and Cooling Performance of a Chiller Heater
The heating and cooling load profile displayed in Figure 5.2.1.6 for Straumann USA is
used to determine whether a chiller heater would be applicable for the building. The
load profile shows that most of the large heating demand occurs when the cooling load
is around 400 tons. This poses a bit of a problem because the chiller of 500 tons will be
operating at nearly 80% of full capacity. By using the heating and cooling graph in
Figure 6.2-5, only 20% of the total heating capacity of the chiller heater can be used.
Nearly 2400 MBH or more of heating capacity is needed and at this operation point only
1200 MBH is available. A boiler is still necessary for over half of the heating capacity.
While some heating capacity is better than none at all, new boilers are not needed for
Straumann USA so there is not additional expense to use the boilers. The boilers are
also more efficient at heating than the chiller heater so unless a boiler would need to be
replaced and the chiller heater could prevent the purchase of an additional boiler, there
does not seem to be any additional benefit from using a chiller heater in this application.
A full analysis of the heating and cooling load profiles resulted in determining that
heating is needed 3222 hours during the year at Straumann USA. The chiller heater
would be available for combined heating and cooling in only 733 of those hours or 23%
of the time. Of the hours a chiller heater could be used nearly one third of the time, 226
hours, a supplement boiler would be necessary to meet the heating load. Overall the
chiller heater would only be able to meet the full heating demands of Straumann USA
16% of the time heating is needed.
- 22 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Cooling and Heating Loads
Cooling and Heating Load (12,000 Btus)
1200
Cooling
Heating
1000
800
600
400
200
0
0
1000
2000
3000
4000
5000
6000
7000
8000
Hour
Figure 6.2-6: Simultaneous Heating and Cooling Load
6.2.2 Free Cooling Options
There are several opportunities with any mechanical system to reduce energy
consumption, and save annual operating costs. One way to do so is to include
waterside free cooling. When a building is experiencing reduced load conditions, and
low wet bulb temperatures exist, it is possible to reject heat from the chilled water loop
without the use of a chiller. Any hour that the chiller is turned off, significant amounts of
energy can be saved, since a chiller is typically one of the largest energy consuming
pieces of equipment. Depending on the number of hours waterside free cooling can be
utilized, a building owner can receive a significant reduction in the yearly energy costs.
Two main types of water side free cooling exist: direct and indirect. Direct free-cooling
simply allows the chilled water return to bypass the chiller and directly enter the cooling
tower where it is cooled and supplied to the loads at the chilled water supply
temperature. The main disadvantage of this type of free cooling is that debris can enter
the chilled water system through the cooling tower. The second major type of waterside
free cooling, which is present in at Straumann USA, is the indirect method. In this
configuration, the chilled water and condenser water loops remain separated. Heat is
transferred from the chilled water line to the condenser water line typically through a
plate and frame heat exchanger. While this type of waterside free cooling requires the
- 23 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
additional first cost of a plate heat exchanger, it makes certain no debris from the
cooling tower enters the chilled water loop where cooling coils could possibly get
clogged.
While there are two types of waterside free cooling, indirect free cooling can also be
piped in one of two ways. The first is a parallel arrangement in which the heat
exchanger utilized for free cooling is placed in parallel with the chiller, refer to Figure
6.2-7 for a schematic of the system.
Figure 6.2-7: Parallel Waterside Free Cooling Schematic
This seems to be the most common way an indirect system is piped. In this
arrangement, waterside free cooling can only be utilized when it can reject enough heat
to produce the chilled water supply temperature. Any conditions that do not reject the
entire load of the chilled water loop require the operation of the chiller
The second piping configuration for an indirect free cooling system is a series
arrangement. In this layout, the heat exchanger is placed in series with the chiller, refer
to Figure 6.2-8 for a schematic of the system.
- 24 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Figure 6.2-8: Series Waterside Free Cooing Schematic
In such an arrangement, free cooling can be used as soon as the cooling tower is able
to produce condenser water that is below the chilled water return temperature. This
allows waterside free cooling to be used more hours each year. It is not necessary for
the condenser water loop to reject the all the heat from the chilled water loop. In this
configuration, free cooling can be utilized to pre-cool the chilled water return before it
enters the evaporator, resulting in a lower load seen by the chiller. When the condenser
loop is able to reject all the heat from the chilled water loop, the chiller can be turned off
and the system will operate just like a parallel piping arrangement. Some
disadvantages of the series system include more advanced controls, and an extra pump
on the condenser water loop.
As previously discussed, waterside free cooling is most effective in climates with a low
wet bulb temperature. Figure 6.2-9 shows the predicted wet bulb distribution used by
Carrier’s Hourly Analysis Program for a year in Andover Massachusetts.
- 25 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Number of Hours per Wet Bulb Range
2500
Number of Hours
2000
1500
1000
500
0
WB < 30
30 < WB < 40 40 < WB < 50 50 < WB < 60 60 < WB < 70
WB > 70
Wet Bulb Temperature (F)
Figure 6.2-9: Yearly Hours per Wet Bulb Range
While it can be seen that over half of the hours in Andover have a wet bulb temperature
of less than 50°F, the actual hours where cooling is necessary may not occur during the
times of the low wet bulb temperature. Such low temperatures may or may not be
capable of providing free cooling depending on the size of the load. Figure 6.2-10
displays the hourly load with the corresponding wet bulb condition. As can be seen by
the load distribution, it appears that a load of 250 – 350 tons (slightly less than 50% of
the design load) is most common at wet bulb temperatures below 40°F. Based on this
comparison of loads and wet bulb temperature, it can be assumed that such a building
might be able to effectively utilize waterside free cooling since there are quite a few
hours with low wet bulb temperatures and reduced loads.
- 26 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Cooling Load vs Wet Bulb Temperature
Cooling Load (tons)
800.0
600.0
400.0
200.0
0.0
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
Wet Bulb Temperature (F)
Figure 6.2-10: Straumann Load vs Wet Bulb Temperature
In order to analyze the two types of waterside free cooling, the Engineering Equation
Solver (EES) program will be used to calculate the power required at each load coupling
a cooling tower, chiller, and heat exchanger. The basis for the cooling tower and chiller
models are taken from AE 557, a Penn State course in central cooling systems.
Parallel Waterside Free Cooling Operation
A heat exchanger is modeled between the chiller and cooling tower for the parallel
configuration. The cooling tower runs at 100% fan operation until the condenser water
temperature reaches 60°F. If the tower is capable of producing condenser water at less
than 60°F the fan is modulates between off and full speed to maintain 60°F condenser
water. Refer to Figure 6.2-11 for a schematic of the system operation under such
conditions. Portions of the system that are “off” or have no flow are shown in grey.
Figure 6.2-11: Parallel System Chiller Mode
- 27 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
If the cooling tower is capable of producing condenser water at temperature low enough
to completely reject heat from the chilled water system, the chiller is then turned off.
During this operation period, the cooling tower modulates between full speed and off to
supply condenser water that maintains 45°F chilled water leaving the heat exchanger.
Refer to Figure 6.2-12 for a schematic of the heat exchanger operation in the parallel
system. If 45°F chilled water can not be produced by full speed fan operation, the
chiller will turn back on until such conditions can again be met.
Figure 6.2-12: Parallel System Free Cooling Mode
Series Waterside Free Cooling Operation
The series heat exchanger model is slightly more complicated. Once again, the cooling
tower runs with fans at 100% until it produces a condenser water temperature of 60°F
for the chiller while bypassing the heat exchanger. When a full speed fan is able
produce condenser water temperatures between 55°F and 60°F the fan is modulated
between 100% and off to maintain 60°F condenser water for the chiller. Refer to Figure
6.2-13 for a schematic of this operation mode.
- 28 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Figure 6.2-13: Series System Chiller and Free Cooling Mode
Once the tower is able to produce condenser water that is lower than 55°F, the heat
exchanger is no longer bypassed. The condenser water first passes through the heat
exchanger to pre-cool the chilled water before entering the evaporator. The condenser
water that leaves the heat exchanger will then mix with some of the water recirculated
from the condenser to maintain 60°F entering the chiller. Refer to Figure 6.2-14 for a
schematic of this type of operation.
Figure 6.2-14: Series System Chiller and Free Cooling Mode
The condenser water system continues to operate in the combination chiller and heat
exchanger mode until the chilled water leaving the heat exchanger reaches 45°F. At
this point the chiller turns off, and the condenser water system operates in a full
waterside free cooling mode just like the parallel arrangement. Figure 6.2-15 displays a
schematic of this type of operation.
- 29 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
Figure 6.2-15: Series Free Cooling Mode
The results for the two types of waterside free cooling are summarized in Tables 6.2-2
and 5.2-3. One of the surprising results is that free cooling without a chiller can only be
used 2 hours of the entire year for the Straumann facility. Another interesting result is
that if a series free cooling system is turned on as soon as condenser water can be
produced below 55°F, it will use more annual energy than a free cooling system in a
parallel arrangement.
Heat Exchanger in Parallel
No Cooling
Free Cooling
Chiller Cooling
Total
Hours
Ton
Hours
5051
2
3707
8760
0
591
904743
905334
Fan Energy Chiller Energy
(kW)
(kW)
0
50
103773
103822
0
0
665889
665889
Additional
Pump (kW)
Total Energy
(kW)
0
0
0
0
0
50
769662
769711
Table 6.2-2: Summary of Parallel Free Cooling Results
Heat Exchanger in Series (55F)
No Cooling
Free Cooling
Series Cooling
Chiller Cooling
Total
Hours
Ton
Hours
5051
2
108
3599
8760
0
591
33036
871707
905334
Fan Energy Chiller Energy
(kW)
(kW)
0
50
3222
103773
107044
0
0
11480
651008
662487
Additional
Pump (kW)
Total Energy
(kW)
0
0
934
0
934
0
50
15635
754780
770465
Table 6.2-3: Summary of Series Free Cooling Results
- 30 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
After finding that a series free cooling system could actually increase the amount of
energy a chilled water plant consumes annually, the series free cooling is optimized to
reduce the annual energy to a minimum. In order to operate the system in a way the
consumes the least energy, the series free cooling system should operate in a series
cooling mode (operating both the heat exchanger and chiller) until the condenser water
temperature can be produced at 51°F. Prior to this temperature the system should
maintain a condenser water temperature of 60°F and operate only the chiller. The
results of the 51°F series free cooing are summarized in Table 6.2-4.
Heat Exchanger in Series (51F)
No Cooling
Free Cooling
Series Cooling
Chiller Cooling
Total
Hours
Ton
Hours
5051
2
38
3669
8760
0
591
11470
893274
905334
Fan Energy Chiller Energy
(kW)
(kW)
0
50
1134
103773
104956
0
0
12726
651008
663734
Additional
Pump (kW)
Total Energy
(kW)
0
0
324
0
324
0
50
14184
754780
769014
Table 6.2-4: Summary of Optimized Series Results
The three different waterside free cooling systems are compared in Table 6.2-5. The
results show that even when optimizing the series free cooling system for Straumann
USA, only a minimal savings of 698kW can be expected over the course of a year, while
a series free cooling system starting to operate at a condenser water temperature of
55°F will actually consume more energy.
Ton Hours
Free Cooling
Parallel
Series (55)
Series (51)
591
591
591
Ton Hours Series Total Energy Savings Compared to
Cooling
(kW)
Parallel (kW)
0
33036
11470
769711
770465
769014
-754
698
Table 6.2-5: Summary of Free Cooling Results
The results are particularly interesting. Even though the at first it was assumed that the
weather and loading conditions for Straumann USA would result in good application for
waterside free cooling, the results tell a different story. It is very possible that a under
some conditions, a series free cooling system can actually consume more energy than
just running the chiller. When the condenser water is only slightly below the chilled
water return temperature, the pre-cooling of the chilled water is minimal. Under such
conditions, this means the chiller would require almost as much energy with the slight
pre-cooling as without it. The overall increased energy is caused by increases in fan
and pumping energy. When both the chiller and heat exchanger are in operation two
pumps are running rather than just one. The fan energy of the cooling tower would also
- 31 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
increase in order to produce condenser water temperature below 60°F. By minimizing
the total energy consumed during potential free cooling hours, it is found that waiting to
use free cooling until 51°F condenser water can be produced, the savings in chiller
energy outweighs any additional pumping and fan costs.
Based on the results, a plot of cooling load versus wet bulb temperature for each
operation mode is displayed in Figure 6.2-16. This shows that for Straumann USA to
use free cooling alone, the wet bulb temperature must be less than 6°F. The optimal
series cooling can be use when the wet bulb temperature ranges from 6°F to 15°F. Any
temperatures above this will solely require a chiller to reject heat from the building
chilled water system.
Cooling Load vs Wet Bulb Temperature
Cooling Load (tons)
800.0
600.0
400.0
200.0
0.0
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
Wet Bulb Temperature (F)
Chiller
No Cooling
Series Cooling (51)
Free Cooling
Chiller 2
Figure 6.2-16: Cooling Load vs Wet Bulb Temperature at Each Cooling Mode
6.3 Mechanical Conclusions
The analysis of the Straumann USA facility provided some very interesting results.
When comparing the airside systems the DOAS system saves on annual energy costs.
When comparing the direct-fire absorption and electric centrifugal chillers with the same
airside system, the absorption chiller resulted in a higher annual energy cost. However,
an absorption/DOAS system did result in a lower annual energy cost than an
electric/VAV system. When considering using the absorption chiller to both
simultaneously produce hot and chilled water it is found that the heating load for
Straumann USA would only be met 16% of the time. Since boilers are already present,
- 32 -
Kevin Kaufman
Straumann USA
Mechanical Option
Andover, MA
Faculty Consultant: Dr. Bahnfleth
______________________________________________________________________
there would be no reduction boiler size for the facility so no initial cost savings would be
a factor. An analysis of the waterside free cooling capabilities of Straumann USA also
provided some interesting results. While a few additional hours of free cooling can be
obtained by using a series free cooling arrangement, it must be carefully controlled to
prevent the cooling costs from actually increasing if condenser water is supplied
between 51°F and 55°F.
- 33 -